1. Field of the Invention
The present invention relates to a series control type regulator which can decrease rapidly an output voltage when operation of a transistor controlling the output voltage is stopped.
2. Description of the Prior Art
FIG. 1 shows a circuit of a conventional series control type regulator. Referring to FIG. 1, a first transistor Q1 is connected in series between an input terminal 1 and an output terminal 2 and a detection circuit 10, which comprises resistors R1 and R2 connected in series between the output terminal 2 and a ground, for detecting an output voltage V.sub.OUT.
A node of the resistor R1 and R2 of the detection circuit 10 is connected to an inverted input terminal (-) to supply a detecting voltage corresponding to the output voltage V.sub.OUT to an error amplifier circuit 3. A non-inverted input terminal (+) of the error amplifier circuit 3 is connected to a reference voltage source to generate an output signal corresponding to the amount of deviation between a reference voltage V.sub.REF and the detecting voltage. An output terminal of the error amplifier circuit 3 is connected to a base of the transistor Q1 to control a base current of the transistor Q1 by means of the output signal of the error amplifier circuit 3. Thereby the transistor Q1 can change impedance between a collector and an emitter thereof according to the output signal of the error amplifier circuit 3 to control the output voltage V.sub.OUT. By this operation of the transistor Q1, a predetermined output voltage V.sub.OUT can be obtained at the output terminal 2.
An output capacitor C1 is connected between the output terminal 2 and the ground to prevent a rapid change of a load and a noise generated in a regulator or the load from affecting badly the output voltage V.sub.OUT . A capacitor having a large capacitance is usually used for the capacitor C1.
A switching circuit 4 and a switch 5 are connected between the input terminal 1 and the error amplifier circuit 3 and the transistor Q1 is set to a state of operation or a state of stop of operation through the switching circuit 4 and the error amplifier circuit 3 by an ON and OFF state of the switch 5.
An input voltage V.sub.IN of the input terminal 1 is supplied from an external dc source E1, but a main switch for connecting the dc source E1 to the input terminal 1 is omitted in the drawings.
In the circuit as shown in FIG. 1, when the switch 5 is ON, the transistor Q1 becomes an state of operation and a predetermined output voltage V.sub.OUT is obtained at the output terminal 2 by control operation of the transistor Q1. By any reason, however, in case of stopping a supply of the output voltage V.sub.OUT to a load from the series control type regulator, operation of the transistor Q1 is stopped rapidly by switching off the switch 5.
Setting the transistor Q1 to the state of stop may be carried out, for example, in a way of cutting off a bias current supplied to the error amplifier circuit 3 by the switching circuit 4 operating according to the switch 5, thereby stopping operation of the error amplifier circuit 3 to turn off the transistor Q1. Japanese Patent Application No. 7-86119 (Unexamined Patent Application Publication No. 8-255028) proposed by the same inventor as that of the present application discloses a concrete example of such a way.
Such a series control type regulator has been used as a power supply of a various of circuits or electronic apparatuses, and in particular, has been used extensively in these fields with recent development of portable digital communication apparatuses.
In recent digital communication apparatuses, in many cases, transmissions and receptions are performed rapidly repeatedly. For this, the regulator used as a power supply of the electric communication apparatuses is required to rise up and fall down the output voltage V.sub.OUT at high speed. However, many technologies for rising it up at high speed are known, but few technologies for falling it down at high speed are known.
In the circuit of FIG. 1, in case of setting the transistor Q1 to the state of stop to fall down the output voltage V.sub.OUT, it will decrease rapidly if a load is heavy. In a light load, however, it will take a longer time than in the heavy load to decrease the output voltage V.sub.OUT. This is because a discharge time of the output capacitor C1 becomes short if the load is heavy and becomes long if it is light. For example, we assume that although operation of a transistor for controlling an output voltage in the regulator stops, a receiving circuit connected to the regulator is still in operation because a falling speed of the output voltage is slow. In this case, if operation of a transmitting circuit starts, a howling phenomenon will be generated between the receiving circuit and the transmitting circuit, and the receiving circuit might be damaged.